Post on 01-Jun-2018
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Short Channel EfectsShort Channel Efects
in MOSFETsin MOSFETs
Fabio DAgostinoFabio DAgostino
Daniele QuerciaDaniele Quercia
Fall, 2000Fall, 2000
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PresentationPresentation Outline
Short-Channel Devices
Short-Channel Effects (SCE)
The modification of the threshold voltage due to SCE
A numerical example
Simulation: SCE impacts on the threshold voltage
Simualtion: limiting effect of the saturation velocit
Conclusion
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DenitionDenition
What is a short-channel device?
A !"S#ET is considered to $e short %hen the channellength is the same order of magnitude as the depletion-laer %idths (xdD& xdS)
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Short Channel EffectsShort Channel Effects
Five different h!sical henono"ena have to be
considered in short-channel devices#
Drain induced $arrier lo%ering and 'unchthrough
Surface scattering
elocit saturation
mpact ioni*ation
+ot electrons
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Drain-induced barrier loweringDrain-induced barrier lowering
(DI!"(DI!"
The electrons (carriers) in the channel face a potential
$arrier that $loc,s their flo%s
The potential $arrier& in small-geometr !"S#ETs& is
controlled $ a t%o-dimensional electric field vector (inother %ords $ $oth Sand DS)
f the drain voltage is increased the potential $arrierin the channel decreases& leading to
Drain-$nduced %arrier &o'ering (D$%&)
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Drain-induced barrier lowering (DI!" andDrain-induced barrier lowering (DI!" and
PunchthroughPunchthrough
.nder D/0 condiction electrons can flo% $et%een the
source and drain even if S 1 T
The channel current that flo%s in this case is calledsu$threshold current
*unchthrough
The D/0 phenomenon can $e accompanied $ the so-
called punchthrough& that occurs %hen the depletionregion surrounding the drain extends to the source
'unchthrough minimi*ed %ith thinner oxide& larger
su$strate doping (and longer channel2)
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Sur#ace scatteringSur#ace scattering
#or small-geometr !"S#ETs& the electrons mo$ilit in
the channel depends on a t%o-dimensional electric field(+& !)
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Sur#ace scatteringSur#ace scattering
The surface scatteringoccurs %hen electrons areaccelerated to%ard the surface $ the verticalcomponent of the electric field +
The collision of the electrons causes a reduction in themo$ilit
Electrons moves %ith great difficult parallel to the
interface
The average surface mo$ilit is a$out half as muchas that of the $ul, mo$ilit
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$elocit% saturation$elocit% saturation
#or lo% !the electron drift velocit vdein the channel
varies linearl %ith the electric field intensit
As !increases a$ove 3456cm the drift velocit tends
to approach a saturation value of vde(sat)7348cm6s around
!73496cm
The velocit saturation reduces the transconductanceof short-channel devices in the saturation condiction&as the follo%ing formula sho%s:
gm7 Coxvde(sat)
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I&'act ioniationI&'act ioniation
he resence of high longitudinal fields can
accelerate electrons that "a! be able of ioniing .iato"s b! i"acting against the"
;ormall most of the e- are attracted $ the drain& soit is plausi$le a higher concentration of holes near thesource
f the holes concentration on the source is a$le tocreates a voltage drop on the source-su$strate n-p
thene- ma $e in
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)ot electrons)ot electrons
he channel /ot lectronseffect is caused b!
electrons flo'ing in the channel for large 1D.
e- arriving at the Si-Si"?interface %ith enough ,inetic
energ to surmount the surface potential $arrier are
inected into the o+ide
This ma degrade permanentl the C- characteristicsof a !"S#ETs
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The &odication o# theThe &odication o# the
threshold *oltagethreshold *oltagedue to short-channeldue to short-channel
efectsefects
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
3uation giving the threshold voltage atero-bias
( )ox
IFASi
ox
FFBTCqDNq
CVV +
++= 22120
accurate for large !"S transistors
not accurate for short-channel !"S transistorsthe amount of $ul, charge is overestimated
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
&arge 45. transistor# the delition isonl! due to the electric fieldcreated b! the gate voltage6
."all-geo"etr! transistor# in addition tothe revious contribution, thedelition charge near n7 regions isinduced b! -n unctions6
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
he bul8 deletion charge is s"aller thane+ectedthe threshold voltage e+ression "ust be"odified to account for this reduction#
00)(0 TTchannelshortT VVV =
10# thresholdvoltage shift
10# ero-bias
thresholdvoltage
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
We find the follo'ing relationshi#
( ) ( )222DjdmdDj Lxxxx ++=+
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
9 and solving for LD
we obtain#
'here
( )
+++= 1
212
222
j
dDjdDjdDdmjjD
x
xxxxxxxxL
( )0
2
+
= DSASi
dD
VqN
x
.i"ilarl!, the length &.can also be found as
follo's#
'here( )0
2
=
A
SidS
qN
x
+ 1
21
j
dSjSx
xxL
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
he a"ount of the threshold voltage reduction
1,0due to short-channel effects can be foundas#
++
+= 1
211
21
222
10
j
dS
j
dDj
FASi
ox
Tx
x
x
x
L
xNq
CV
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
e consider an n-channel !"S process %ith the follo%ingparameters:
=su$strate doping densit :A;&=gate oxide tic,ness to+; 0 n"&
=oxide-interface fixed charge densit :o+;@6
n addition& %e assume that the channel region is implanted%ith p-tpe impurities(impurit concentration :$; 2
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
We obtain 9
VT0
= 0.855V - VT0
;
where ___________
VT0= ( 0.343/L[m] ) * (-0.724 + ( 1 + 2 xdD) _________________
xDd= 0.13 (0.76 + VDS)
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Modication o# $Modication o# $T)T)due to SCEdue to SCE
9 and lotting the variation of the thresholdvoltage 'ith the channel lenght
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Vth:T
hreshold
voltage
[V]
(Vth vs. L) @ Vds=1V [-----] Vds=
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Si&ulation+ i&'act o# SCE on the thresholdSi&ulation+ i&'act o# SCE on the threshold
*oltage*oltage
e simulate four n!"S#ETs in parallel& %ith differentchannel lengths and 'idths
All the transistors have the same parameters@ 0EE0 ?
of 'spice is used
#or each transistor %e generate the D-Scharacteristic at DS 7 4=3
The plots sho% ho% devices %ith smaller geometr havehigher drain currents at the same gate-to-sourcevoltage (i=e=& smaller threshold voltages)
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Si&ulation+ i&'act o# SCE on the thresholdSi&ulation+ i&'act o# SCE on the threshold
*oltage*oltage
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Si&ulation+ the li&iting efects o# the saturationSi&ulation+ the li&iting efects o# the saturation
*elocit%*elocit%
e simulate t%o n!"S#ETs in parallel& %ith the samechannel length and %idth
"ne transistor has a limited saturation velocit of vde(sat)
7 2C
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Si&ulation+ the li&iting efects o# the saturationSi&ulation+ the li&iting efects o# the saturation
*elocit%*elocit%
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ConclusionConclusion
.E are governed b! co"le+ h!sical heno"enathat can be "ainl! related to the
$nfluence of both vertical and horiontal
electric field co"onents on the flo'of the electrons in the channel
suall! .E interacts the one 'ith the other
.E should be carefull! considered in order toevaluate their i"act on the general behaviour ofthe device, both for short-ter" and long-ter"